Aromatics extraction process

ABSTRACT

A cracked gasoline which is to be used in an aromatics extraction process is pretreated by distilling out a distillate boiling up to 90*C, heating the residue to 200* to 350*C, distilling at a second distillate boiling up to 250*C and hydrogenating this distillate.

United States Patent 1191 Rowe et al. Aug. 12, 1975 AROMATICS EXTRACTION PROCESS [56] References Cited [75] Inventors: Frederick Rowe; Michael George UNITED STATES PATENTS NW0"; Anthony Fairweatheri 3,162,599 12/1964 McAnhur et al. .1 208/255 all Of stoekton-en-Tees, England 3,190,830 6 1965 Rowland et al 208/143 3,494,859 2/1970 Parkerm; 208/210 [73] Asslgnee' 'P F Chemlcal lndusmes 3,751,361 8/1973 Caflisch et a1. 208 48 R Llmlted, London, England Filedi 1973 Primary ExaminerHerbert Levine [21] APP] 340 923 Attorney, Agent, or FirmCushman, Darby &

Cushman [30] Foreign Application Priority Data 57 ABSTRACT Mar. 13, 1972 United Kingdom 11593/72 1 A cracked gasoline which is to be used in an aromatics [52] US. Cl. 208/92; 208/71; 208/96; xtr ti n process is pretreated by distilling out a dis- 208/216; 208/217; 208/254; 208/255 tillate boiling up to 90C, heating the residue to 200 [51] Int. Cl Cl0g 37/08 to 5 C, istilling at a second distillate boiling up to [58] Field of Search 208/92, 48, 143, 144, 255, 5 C nd ydrogenating this distillate.

- 10 Claims, No Drawings AROMATICS EXTRACTION PROCESS THE PRESENT lNVENTlON relates to the treatment of a hydrocarbon feedstock for an aromatics extraction process, and in particular, to the treatment of an aromatics-containing cracked gasoline to enhance its suitability for use in such a process.

Aromatic hydrocarbons such as benzene. toluene and the three isomeric xylenes are important chemicals which are largely petrochemical in orgin. They are usually produced by known reforming processes from the naphtha fraction (boiling range 32 to 205C) obtained in the distillation of crude petroleum and may be recovered from the reformed fraction by a variety of extractive techniques usually after sulphur and nitrogen compounds present have been hydrogenated. Alternatively the naphtha may first be subjected to a thermal or steam cracking operation to produce ethylene and propylene together with a number of higher olefinically unsaturated hydrocarbons such as butadiene and isoprene. The cracking process is followed by a series of distillations to separate the cracked product into a number of fractions comprising hydrocarbons of similar boiling points and the C to C aromatic hydrocarbons are generally contained in a fraction, commonly referred to as a cracked gasoline, together with C and higher alkanes, alkenes, cycloalkanes and cycloalkenes.

The cracked gasoline may also be subjected to an aromatics extraction process after the lower boiling C hydrocarbons have been removed usually by distillation. a process often referred to as depentanization. Again it is necessary to hydrogenate sulphur and nitrogen compounds present in the depentanized cracked gasoline prior to the extraction process and this is usually accomplished at an elevated temperature using molecular hydrogen and a catalyst such as cobalt and molybdenum supported on alumina. We have found, however, that other components in the depentanized cracked gasoline have an adverse effect on the catalyst activity and life and an objective of the present invention is to so treat the depentanized cracked gasoline that the catalyst activity and life are improved.

According to the invention a process for the treatment of a cracked gasoline prior to the recovery of aromatic hydrocarbons therefrom includes the steps:

a. distilling the cracked gasoline to recover a first distillate fraction boiling up to 90C, suitably up to 80C at atmospheric pressure,

b. heating the residue from this distillation in the liquid phase at a temperature in the range 200 to 350C.

c. distilling the heated product to separate a second distillate fraction boiling up to 250C at atmospheric pressure, and

d. hydrogenating said second distillate fraction.

The second distillate fraction from step (d) is suitable for the extraction of aromatic hydrocarbons therefrom.

The cracked gasoline is derived from a thermally or steam cracked petroleum fractions which may be naphtha (boiling range 32to 205C) or gas oil (boiling range 205 to 430C). The gasoline is separated from the cracked product by distillation and typically boils in the range 0 to 250C at atmospheric pressure and may contain the following hyrocarbons isoprene, cis and trans-piperylene, n-pentane, isopentane, pentenel, cyclopentadiene, dicyclopentadiene, trans-pentene- 2, Z-methylbutene-Z, cyclopentcne, cyclopentane, benzene, toluene, ethylbenzene, o-mand p-xylene, npropylbenzene. 2-, 3- and 4-ethyl-toluene, mesitylene, pseudocumene and styrene.

The cracked gasoline is distilled usually at atmospheric pressure so as to produce a distillate boiling up to 90C, suitably up to C containing the hydrocarbons listed above from isoprene through to cyclopentane together with a little benzene and a residue containing the major part of the benzene together with the remaining hydrocarbons listed and boiling, for example, in the range 80 or to 250C. In practice the distillation is usually carried out in a continuous manner in a distillation column or columns, the cracked gasoline being continuously introduced to the approximate mid-point of the column and the two fractions being continuously removed from the top and bottom of the column respectively. The residue from the bottom of the column is known for convenience in this specification as depentanized cracked gasoline.

The depentanized cracked gasoline is heated in stage (b) to a temperature in the range 200 to 350C under sufficient pressure to maintain it in the liquid phase. This is suitably the autogenous presssure at the temperature of the heating and is usually 300 to 600 p.s.i.g. Preferably the temperature is 250 to 300C and, preferably, the time of heating /2 to 4 hours. If desired the heating may be carried out batchwise in a stirred pot but is preferably carried out continuously, e.g. during passage through a tubular reactor.

The product from the heating stage is distilled, usually at atmospheric pressure so as to produce the second distillate fraction boiling up to 250C, preferably up to 180C at l at, and leaving a polymeric high boiling residue. This distillation is also preferably carried out continuously in one or more distillation columns, e.g. in a first flash distillation column followed by a second, fractional distillation column.

The second distillate fraction may be hydrogenated in the liquid or vapour phase with molecular hydrogen over a suitable catalyst. The catalyst may be one or more metals such as iron, cobalt, nickel, tungsten, molybdenum, platinum or palladium as, e.g. the metal itself or its oxide or sulphide supported on an inert support such as alumina or silica, e.g. cobalt and molybdenum on alumina, or platinum on alumina. If desired a two-stage catalyst may be used, e.g. a first stage in which cobalt or nickel plus molybdenum on alumina or palladium on alumina is used and a second stage in which the catalyst is platinum on alumina or cobalt and molybdenum on alumina. The hydrogenation is preferably carried out at C to 600C at 300 to 500 p.s.i.g. pressure of hydrogen. In a preferred two-stage hydrogenation the first stage may be liquid phase at 100 to 300C, preferably to 250C and the second stage 'vapour phase between 300 and 600C, preferably 350 to 550C.

The hydrogenated distillate fraction from step (d) is a suitable condition for the extraction of its aromatic content and any of a number of known aromatics extraction processes may be used. There processes employ one or more solvents which selectively extract the aromatics, notably benzene, toluene and xylenes from the distillate fraction. The aromatics are then recovered from the solvent by normal or extractive distillation. Preferred extractive solvents include diethylene glycol and/or dipropylene glycol with a small amount of water, sulpholane, N-methyl-pyrrolidone, dimethyl sulphoxide or propylene carbonate while less favoured extractive solvents are silver fluoborate, N- methylformamide, polyalkylene glycols, morpholine or alkyl carbamates. The extraction may take place at temperatures in the range ambient to 100C under atmospheric pressure or a positive pressure, e.g. up to 200 p.s.i.g.

In a preferred form of the process a cracked gasoline boiling range 10 to 220C at l at. derived from a steam I cracked naphtha is distilled in a continuously operated distillation column of 10 to 15 theoretical plates, the column head temperatue being 65C and the base temperature 150C. From this column a distillate is obtained boiling range 10 to 80C at l at. comprising acyclic and cyclic alkanes and alkenes containing 5 or 6 carbon atoms. The residue from the column boiling range 80 to 220C at 1 at. contains the desired benzene, toluene and xylenes together with other aromatic hydrocarbons and styrene. This residue is then fed continuously through a tubular reactor in which its temper ature is maintained at 270 to 290C and in which the pressure of 400 p.s.i.g. is sufficient to maintain the depentanised cracked gasoline in the liquid phase. The residence time in the reactor is one hour. On leaving the reactor the heat soaked product is fed directly to a flash vessel and the resulting flash distillate to a second, continuously operated distillation column with a 2 1 reflux ratio and column head and base temperatures of 175 to 185C and 200 to 210C respectively. The distillate from this second column boils at up to 180C at l at. The residue from the flash vessel is a polymeric oil containing higher aromatic hydrocarbons.

The distillate from the second, continuously operated column is heated to 120C, mixed with hydrogen in a molar ratio of 1 l and is then passed over a two-stage hydrogenation catalyst, firstly in the liquid phase over sulphided nickel and molybdenum on alumina at 120 to 150C, and then in a second vapour phase stage over platinum on alumina at 480 to 510C. The pressure in the hydrogenation unit is 350 to 400 p.s.i.g.

Following the hydrogenation the hydrocarbon product may be condensed and fed to an extraction column maintained at 40C in which it passes countercurrent to a stream of sulpholane. The sulpholane extract is separated, purified and the aromatic hydrocarbons recovered therefrom by distillation in a vacuum column maintained at head and base temperatures of 70 and 170C respectively. The aromatic hydrocarbons are receovered as the distillate while the sulpholane taken from the base of the column is recycled to the extraction process.

EXAMPLE 1 A steam cracked gasoline. boiling range 10 to 220C was distilled in a column of theoretical plates to give as residue, a depent-anized cracked gasoline boiling between 80 and 220C.

This depentanized cracked gasoline was passed through a thermal treatment unit which consisted of a tubular steel reactor divided into separately stirred compartments. The flow characteristics of the gasoline through the unit were equivalent to approximately 5 stirred pots in series. The maximum temperature in the unit was 275 to 280C, the pressure 400 p.s.i.g. and the mean residence time 2 hrs.

The thermally treated gasoline was next fed to a flash still in which the pressure was let down to 20 p.s.i.g., thereby flashing off a distillate, boiling range up to 220C, and leaving an oily residue (15% by weight of the feed)v The flash distillate was then distilled in a glass lined still at 2 1 reflux ratio to give a distillate of final boiling point 180C which was condensed and fed to a hydrogenation unit containing a freshly sulphided nickel/molybdenum on alumina catalyst. The inlet temperature to the catalyst bed was 200C. the pressure 300 p.s.i.g. and the hydrogen to hydrocarbon molar ratio 1 l.

A comparative experiment was carried out in which the heat treatment stage was omitted and it was found that in this case after 5 days continuous running the gum content of the hydrocarbon stream leaving the catalyst bed was more than 10 times the amount in the stream when the thermal treatment was applied. The gum contained in the stream causes rapid loss of activity in the second stage of the hydrogenation and also fouls items of equipment such as heat exchangers.

We claim:

1. A process for the treatment of a cracked gasoline prior to the recovery of aromatic hydrocarbons therefrom which includes in combination the steps,

a. distilling the cracked gasoline to recover a first distillate fraction containing all the components in said cracked gasoline boiling up to C at atmospheric pressure and a first residue containing all the components which boil above 80C,

b. heating said residue which boils above 80C from distillation step (a) in the liquid phase at a temperature in the range 200 to 350C to form a substantial amount of material boiling above the end point of said residue.

0. distilling the heated residue from step (b) to separate a second distillate fraction boiling up to 250C at atmospheric pressure, and a second residue containing said material boiling above the end point of the first residue, and

d. hydrogenating said second distillate fraction.

2. The process of claim 1 in which the step (b) heating is carried out in a tubular reactor.

3. The process of claim 1 in which the step (c) distillation is carried out in a flash distillation column followed by a fractional distillation column.

4. The process of claim 1 in which the step (d) hydrogenation is carried out in the presence of a catalyst comprising a metal selected from the group consisting of iron, cobalt, nickel, tungsten, molybdenum, platinum or palladium.

5. The process of claim 4 in which the catalyst is the metal itself or its oxide or sulphide supported on alumina or silica.

6. The process of claim 4 in which a two-stage catalyst is used. 0

7. The process of claim 1 in which the hydrogenation is carried out at to 600C at 300 to 500 p.s.i.g. hydrogen pressure.

8. The process of claim 1 in which the hydrogenated step (:1) product is submitted to an aromatic hydrocarbon extraction process.

9. The process of claim 1 in which:-

a. the first distillate fraction boils up to 80C,

b. the residue in step (b) is heated at a temperature in the range 200 to 300C for V2t0 4 hours,

vapour phase at 300 to 600C in the presence of a catalyst selected from the group consisting of platinum on alumina and cobalt and molybdenum on alumina.

10. The process of claim 9 in which the step (b) hydrogenation product is treated with a selective solvent for the extraction of aromatic hydrocarbons therefrom.

* I l =l 

1. A PROCESS FOR THE TREATMENT OF A CRACKED GASOLINE PRIOR TO THE RECOVERY OF AROMATIC HYDROCARBONS THEREFROM WHICH INCLUDES IN COMBINATION THE STEPS, A. DISTILLING THE CRACKED GASOLINE TO RECOVER A FIRST DISTILLATE FRACTION CONTAINING ALL THE COMPONENTS IN SAID CRACKED GASOLINE BOILING UP TO 80*C AT ATMOSPHERIC PRESSURE AND A FIRST RESIDUE CONTAINING ALL THE COMPONENTS WHICH BOIL ABOVE 80*C, B. HEATING SAID RESIDUE WHICH BOILS ABOVE 80*C FROM DISTILLATION STEP (A) IN THE LIQUID PHASE AT A TEMPERATURE IN THE RANGE 200* TO 350*C TO FORM A SUBSTANTIAL AMOUNT OF MATERIAL BOILING ABOVE THE END POINT OF SAID RESIDUE, C. DISTILLING THE HEATED RESIDUE FROM STEP (B) TO SEPARATE A SECOND DISTILLATE FRACTION BOILING UP TO 250*C AT ATMOSPHERIC PRESSURE, AND A SECOND RESIDUE CONTAINING SAID MATERIAL BOILING ABOVE THE END POINT OF THE FIRST RESIDUE, AND D. HYDROGENATING SAID SECOND DISTILLATE FRACTION.
 2. The process of claim 1 in which the step (b) heating is carried out in a tubular reactor.
 3. The process of claim 1 in which the step (c) distillation is carried out in a flash distillation column followed by a fractional distillation column.
 4. The process of claim 1 in which the step (d) hydrogenation is carried out in the presence of a catalyst comprising a metal selected from the group consisting of iron, cobalt, nickel, tungsten, molybdenum, platinum or palladium.
 5. The process of claim 4 in which the catalyst is the metal itself or its oxide or sulphide supported on alumina or silica.
 6. The process of claim 4 in which a two-stage catalyst is used. 0
 7. The process of claim 1 in which the hydrogenation is carried out at 100* to 600*C at 300 to 500 p.s.i.g. hydrogen pressure.
 8. The process of claim 1 in which the hydrogenated step (d) product is submitted to an aromatic hydrocarbon extraction process.
 9. The process of claim 1 in which:-a. the first distillate fraction boils up to 80*C, b. the residue in step (b) is heated at a temperature in the range 200* to 300*C for 1/2 to 4 hours, c. the second distillate fraction boils up to 180*C at 1 atmosphere pressure, and d. the second distillate fraction is hydrogenated in two stages, the first stage being carried out in the liquid phase at 100* to 300*C in the presence of a catalyst selected from the group consisting of nickel and molybdenum on alumina, cobalt and molybendum on alumina and palladium on alumina, and the second stage being carried out in the vapour phase at 300* to 600*C iN the presence of a catalyst selected from the group consisting of platinum on alumina and cobalt and molybdenum on alumina.
 10. The process of claim 9 in which the step (b) hydrogenation product is treated with a selective solvent for the extraction of aromatic hydrocarbons therefrom. 